The present invention relates to a process for the preparation of cycloaliphatic hydrocarbons by catalytic hydrogenation of aromatic hydrocarbons.
Processes have already been disclosed according to which aromatic hydrocarbons can be converted into the corresponding cycloaliphatic hydrocarbons by catalytic hydrogenation. For example, it is known that benzene can be hydrogenated to cyclohexane, toluene to methylcyclohexane, xylenes to dimethylcyclohexanes, ethylbenzene to ethylcyclohexane, isopropylbenzene to isopropylcyclohexane, naphthalene to tetrahydronaphthalene and/or decahydronaphthalene, and methylnaphthalene to methyltetrahydronaphthalene and/or methyldecahydronaphthalene. In some of these processes the hydrogenation is carried out in the liquid phase whilst in others hydrogenation is carried out in the gaseous phase. Combinations of gas phase and liquid phase hydroganations have also been described.
All previously disclosed processes for the hydrogenation of aromatic hydrocarbons, however, suffer from a series of disadvantages.
As regards the processes which are carried out in the gas phase, the following comments regarding the resulting disadvantages are made in German Patent No. 1,184,756, which relates to a process for the catalytic hydrogenation of aromatic hydrocarbons:
"The processes carried out in the gas phase give in practice only a low yield per unit volume of reaction zone, not only because of the low density of the treated product but also because of the difficulty of cooling this zone effectively. This leads either to the use of bulky apparatus which contain rather large and costly internal cooling pipelines, without however being able to avoid local heating of the catalyst, which reduces its activity, or leads to excessive dilution of the aromatic hydrocarbons, for example with the corresponding hydrogenation products. As a result, recycling installations for the hydrogenation products must be provided, which means that the equipment only provides a low yield." (German Patent No. 1,184,756, column 1, lines 26 to 42).
For example, U.S. Pat. No. 3,146,186 describes a gas phase hydrogenation for the conversion of benzene to cyclohexane in the presence of nickel catalysts, wherein the benzene, in a diluted form, is hydrogenated in two reactors arranged in series, with recycling of unconverted starting material. The performance of this process is rendered difficult by the great expense of the apparatus, the numerous separation and recycling processes, and its high operating pressures.
According to the process of U.S. Pat. No. 3,070,640, aromatic hydrocarbons are hydrogenated at pressures of 300 to 600 p.s.i. (about 21 to 42 bars) in the presence of platinum, nickel, palladium, rhodium, iron and/or Raney nickel, optionally on supports, as the catalyst, two catalyst systems, which differ in their metal content, being employed successively. The conduct of the reaction is also not uniform, since, in order to control the reaction temperature, the process is carried out initially with a diluted catalyst mass, that is to say with a less active catalyst, and subsequently with a more active catalyst (U.S. Pat. No. 3,070,640, column 2, lines 41 to 54). Accordingly, the performance of this process also entails considerable effort.
In carrying out the hydrogenation of aromatic hydrocarbons in the liquid phase, the catalyst is present in a suspended form and must therefore be introduced continuously into the reaction and again discharged continuously from the reaction vessel (DAS (German Published Specification) 1,116,218).
The disadvantages associated with the gas phase hydrogenation and liquid phase hydrogenation of aromatic hydrocarbons also arise in methods which are combinations of the gas phase method and liquid phase method (German Patent No. 1,184,756).
A process for the hydrogenation of aromatic hydrocarbons in the trickle phase is described in DOS (German Published Specification) 1,443,888, wherein the catalysts used are noble metals, especially platinum, rhodium and ruthenium, which are applied to alkaline earth metal carbonates and sulphates, silica, silicates and aluminum oxide as supports. Though only one reactor unit is necessary for the execution of this process, which results in a considerable advantage over the other processes for the catalytic hydrogenation of aromatic hydrocarbons, the process according to DOS (German Published Specification) 1,443,888 is not satisfactory since the catalysts lose their activity after a relatively short period of running and cannot be regenerated. Unsatisfactory working lives have also been recorded for the previously-known hydrogenations which are carried out in the liquid phase and in which catalysts of the Raney type, in a suspended form, are employed (for example DAS (German Published Specification) 1,116,218).
Finally, a further considerable disadvantage of all previously disclosed catalytic processes for the hydrogenation of aromatic hydrocarbons is that they are carried out at elevated pressure. In addition to the patent specification drawn to DAS (German Published Specification) 1,203,257 and U.S. Pat. No. 3,432,565.